1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to elevator systems of the type which are controlled by a speed pattern generator.
2. Description of the Prior Art
It is conventional in traction elevator systems of the type in which the elevator car is responsive to a drive machine which includes a D.C. motor, to control the speed of the D.C. motor, and thus the speed of the elevator car, in response to the error or deviation between a speed pattern signal provided by a speed pattern generator, and a signal responsive to the actual speed of the elevator car.
Predetermined failure modes of the speed pattern generator may result in the elevator car exceeding its rated maximum speed. At a first overspeed magnitude, the governor speed reducing switch operates to reduce the magnitude of the speed pattern signal. At a second overspeed magnitude, an emergency stop is made. If the elevator car reaches a third overspeed magnitude, the safety is set. Thus, it would be desirable to monitor predetermined parameters of the speed pattern signal, such as the magnitude of the speed pattern signal, and the rate of change of the speed pattern signal, before it is applied to the comparator which generates the error signal, and to modify the affect of the speed pattern signal on the comparator when either its magnitude, or its rate of change, or both, exceed predetermined values. Monitoring of the speed pattern signal, and limiting predetermined parameters thereof, however, must be accomplished by monitoring and limiting circuits which have no failure modes which could result in a car overspeed condition.
Further, it would be desirable for the elevator car to approach its rated maximum speed without overshoot, as the speed governor trip settings may be made closer to the maximum rated speed without nuisance trips, when overshooting is not present. Such overshoot of maximum rated speed may be prevented by adjusting the dynamics of the traction drive machine. However, this may be undesirable as it may result in sluggishness of the elevator car during landings. Thus, it would be desirable to be able to prevent overshoot of the maximum car speed, without changing the dynamics of the motor drive, or otherwise deleteriously affecting the performance of the elevator system.